This timepiece date mechanism comprises a units indicator runner equipped with ten teeth or with a multiple of ten teeth and a tens indicator runner equipped with four teeth or with a multiple of four teeth, a transmission connecting the units and tens indicator runners to move the tens indicator runner on by one step for every ten steps of the units indicator runner and a drive runner for driving the units indicator runner, connected to the indicator geartrain of the timepiece.
The dates displayed in the windows of timepieces, particularly wristwatches, are often too small for many users. To overcome this disadvantage, it is possible either to use optical enlargement means or to distribute the numerals of the units and of the tens on the separate display runners so that the magnitude of the numerals is no longer dictated by a one-31st angular portion of the circumference of the display disk along which the numerals are distributed, allowing the numerals to be appreciably enlarged.
There are about four distinct types of mechanism allowing the numerals of the dates displayed to be enlarged.
The first type is represented by documents CH 310559, EP 529191, CH 698671 and EP 1070996. These are mechanisms formed of two disks, one for the tens the other for the units. These disks may be superposed, coaxial or located one on each side of the actuating device which comprises a 31-position wheel indexed by a jumper. This 31-position wheel bears two other wheels, one driving the units with 30 teeth, the 31st being replaced by a corresponding space, the other driving the tens has just four teeth distributed on a circumference of the same diameter as the thirty-one-tooth wheel.
This type of mechanism has several disadvantages. Either the display disks are side by side and two separate windows relatively spaced apart are needed in order not to see the crescent spaces between the edges of the disks, or these disks are superposed and the numerals of the tens and of the units appear at two different levels, which is not pretty and does not make for ease of reading.
This type of mechanism has three jumpers, which means that a relatively high torque is needed to drive the date, correspondingly reducing the torque transmitted to the escapement.
When the drive device has a 31-toothed runner, this occupies a great deal of surface area.
In a second type of mechanism described in CH 324270, FR 2240474 and CH 689601, it has been proposed for only the units disk to be driven, the latter bearing a finger which drives the tens indicator every ten steps.
The disadvantage with this system is that it is necessary to have a 31-position units disk, failing which a significant correction has to be made at the end of each month.
This units disk must be as large as a normal date disk and, in spite of that, the size of the numerals will be limited. This system also entails the date being located at the six o""clock or noon position. This is because, if it is desired for the numerals to be relatively large, it is necessary for them to be oriented radially, which means that they would be lying on their sides if they were to be appear through a window arranged at three o""clock. Finally, in this case the disks are also superposed.
DE 29702749 U1 proposes a very simple display comprising two superposed disks, the upper disk of which comprises the dates from 1 to 15 plus a space, equivalent to an angular space of one date separating the 1 from the 15, in which a window is pierced to reveal the lower disk which bears the dates from 16 to 31. The two disks are actuated in succession, the upper disk by 16 steps, then the lower disk the numerals of which are visible through the window in the upper disk.
The control mechanism is relatively complicated and the display is on two levels, with the aforesaid disadvantages.
The last type of mechanism is illustrated by CH 578202 and CH 690515. It uses two disks which are controlled by Maltese crosses and levers which make this mechanism relatively difficult to adjust.
The object of the present invention is to resolve the drawbacks of the abovementioned solutions, at least in part.
To this end, the subject of the present invention is a date mechanism for a timepiece, as claimed in claim 1.
This mechanism has several advantages, among which mention may be made of the small number of parts, the absence of levers, the presence of just two jumpers, which work together only every ten days, thus reducing the loss of energy from the movement.
The space occupied by this mechanism is small, particularly in terms of surface area, in that it does not use a thirty-one-tooth drive wheel.
All the components of the mechanism can be made by cutting by generation, which is a manufacturing process that makes it possible to obtain very precise geometries with very good surface finishes, which guarantee very good efficiency in the transmission of force in the mechanism.
This makes it possible as a preference to have a mechanism made entirely of circular toothed members with central axes of rotation, something which presents a great advantage, particularly when a date correction is made at around about midnight. The date mechanism can then move in both directions of rotation of these toothed members without any risk of damage or jamming, as may occur with lever-type systems.
Advantageously, the units and tens runners consist of concentric coplanar disks which means that the space between these two disks can be small and of constant width in order not to have to be hidden, allowing the units and the tens numerals to be displayed in a single window.
Bearing in mind the small number of parts and therefore the small bulk of the mechanism, the latter may advantageously be placed entirely under the units and tens indicator disks.
Advantageously, the units disk may have just ten numerals. This disk may be of small size, although the numerals it bears may have a size very much greater than that of a conventional 31-date disk, especially if this disk is coaxial with the center of the movement.